1. Field of the Invention
The present invention relates generally to extrusion processing and, more particularly, to an extrusion apparatus for processing rice bran.
2. Description of the Background
In recent years published medical studies have shown strong links between heart trouble and high cholesterol levels. As a result, consumers have become much more interested in products, especially natural products, which have beneficial effects on cholesterol levels. Rice bran is a low fat product that has been shown to have a favorable effect on cholesterol levels. Consequently there has been an increased demand for rice bran and particularly for rice bran breakfast cereals. Furthermore, there are indications that rice bran oil, which may be used for cooking oil, has the unique ability, under certain conditions, to lower harmful cholesterol levels up to 30 percent without reducing the amount of so-called good cholesterol that protects against heart attacks. Tests with animals show rice bran oil can even raise the good cholesterol levels in some cases.
Although rice is abundantly grown throughout the world, only a small percentage of the rice bran contained within the raw rice has, in the past, been used for human consumption. The fact is that one of the most nutritious foods known to man, rice bran, becomes rancid within a few hours after the milling process, making it substantially inedible by humans after that time and, after several days, digestible only by animals. If the rice bran is to be saved, it must be stabilized almost immediately after milling.
One method of stabilizing the rice bran involves processing it within an extrusion cooker. A rice bran extrusion cooker, which has been used to process the rice bran, is described and illustrated in U.S. Pat. No. 4,741,264 to D. L. McPeak, incorporated herein by reference. By processing the rice bran in the McPeak apparatus, for typically less than one second, the rice bran will often experience a ten fold increase in value due to the fact that it can now be used for human as well as animal consumption. The McPeak apparatus has been used in stabilizing the rice bran so that time is available to extract the rice bran oil and also to utilize the remaining defatted rice bran as high protein, low fat food for human consumption.
However, the McPeak apparatus has a number of serious drawbacks. For instance, the McPeak machines often experience an inordinate amount of vibration during operation. In some cases, the vibration is so severe that even providing a suitable mounting for the machine becomes troublesome. The vibration produces wear in the machine due to increased mechanical strain. Also the vibration indicates inefficiency in that the energy spent on vibration is not being used for processing the rice.
Linked to the vibration problem is a rice bran extruder flow problem in that the rice bran is often extruded from the McPeak apparatus in a turbulent or pulsating flow rather than a smooth flow. A turbulent or pulsating output may result in less consistent cooking of the rice bran and therefore lower quality and a generally short shelf life of the processed rice bran.
Another significant problem which can occur in the McPeak machine concerns difficulty in maintaining a steady temperature on the output. The United States Department of Agriculture generally sets a temperature range within which the cooking temperature must remain. A typical range might be 130 degrees Centigrade plus or minus 4 degrees. The McPeak machine has a hand crank to adjust the extrusion gap and thereby vary the cooking temperature. In some instances, however, it has been observed that a very small change in the extrusion gap quickly results in up to a 10 degree Centigrade temperature change, thus making adjustment of the cooking temperature difficult. The instability of the output temperature is such that it is perplexing, if not impossible, to design an automatic control system adapted to the machine that will control the temperature adequately. This problem also results in a processed bran output that is not always of the best quality due to inconsistent cooking.
Power outages can create difficult problems for a rice bran extrusion machine. If the power to the machine should become cut off, within a short time the liquified bran remaining in the extrusion housing begins to solidify. Even after a short period the extrusion shaft may be effectively glued into the extrusion housing. If the bran solution is allowed to cool inside the housing, a large force, in some cases over twenty tons, may be required for disassembly. An operator may not always be immediately available in time to prevent this problem due to the fact that sometimes these machines run for days at a time making constant observation difficult.
Another problem, which the McPeak patent specification addresses, in column 4, is the problem of providing the extrusion device with a continuous feed of rice bran. Rice bran, due to its oil content, has a tendency to bridge or stick rather than flow freely into the extruder housing. The McPeak machine utilizes a hopper to gravity feed the rice bran into a feed conveyor. However, a gravity feeding hopper is, itself, subject to bridging or sticking of the bran.
A further wear related complication with the McPeak machine is that even though most wear occurs near the output end of the extrusion rotor, the entire rotor, including sections of the rotor without appreciable wear, must be replaced when the output end of the rotor requires replacement.
In summary, the McPeak machine is generally subject to problems of excessive vibration, unstable cooking temperature, reduced efficiency and uncertain reliability.
Consequently, there has been a long felt but unfulfilled need in the rice bran processing industry for an improved apparatus utilizing improvements in extrusion processing techniques for rice bran. The industry has long sought improved rice bran processing apparatus and methods which will result in greater reliability and dependability of operation at reduced levels of capital investment and energy consumption.